i386/testsuite: Add testcase for fixed PR [PR51492]

	PR target/51492

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr51492.c: New test.

gcc/testsuite/gcc.target/i386/pr51492.c

+/* PR target/51492 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -ftree-vectorize -msse2" } */
+
+#define SIZE 65536
+#define WSIZE 64
+unsigned short head[SIZE] __attribute__((aligned(64)));
+
+void
+f(void)
+{
+  for (unsigned n = 0; n < SIZE; ++n) {
+    unsigned short m = head[n];
+    head[n] = (unsigned short)(m >= WSIZE ? m-WSIZE : 0);
+  }
+}
+
+/* { dg-final { scan-assembler "psubusw" } } */
+/* { dg-final { scan-assembler-not "paddw" } } */